Sulfonated poly(phenylene sulfide) films as polyelectrolyte membranes

ABSTRACT

Proton conducting membranes are made of sulfonated films comprising poly(arylene sulfide), an olefinic polymer, and an elastomer. They are used in PEM fuel cells operating at temperatures above 95° C., or at low relative humidity. According to methods of the invention, sulfonated poly(phenylene sulfide) (SPPS) films are provided with a wide range of physical properties, which depend in part on the ion exchange capacity of the films. In particular, the degree or level of sulfonation can be tailored by adjusting reaction conditions such as temperature and time.

INTRODUCTION

The present invention relates to polyelectrolyte membranes and their useas proton exchange membranes in fuel cells. More particularly, theinvention relates to treatment of poly(phenylene sulfide) films toprovide a polyelectrolyte for fuel cell applications.

Fuel cells are increasingly being used as power sources for electricvehicles and other applications. An exemplary fuel cell has a membraneelectrode assembly with catalytic electrodes and a membrane formedbetween the electrodes. Hydrogen fuel is supplied to the anode side ofthe assembly, while oxygen is supplied to the cathode. The membraneprovides an electrical connection between the anode and cathode, andprovides a medium through which fuel oxidation products are transportedfrom the anode to combine with the reduced oxygen at the cathode. Theoverall reaction in the fuel cell is the combination of hydrogen andoxygen to yield water and an electromotive potential. Because theoxidation product of the fuel is essentially H⁺ or a proton, thepolyelectrolyte membrane is also known as a proton conducting membraneor a proton exchange membrane (PEM).

Water management is important in a PEM fuel cell. A proton must behydrated to pass through the proton exchange membrane to combine withoxygen at the cathode. According to various models, 7 to 11 watermolecules are needed to account for the transport of one proton throughthe membrane. It has been observed that when relative humidity in thefuel cell is low, proton transport is less efficient and the currentavailable from the cell is reduced. To avoid this, it is possible tohumidify fuel cells to prevent the membranes from drying out. However,as temperature of fuel cell operation increases, pressurization may benecessary which leads to added expense.

PEM fuel cells operate at temperatures up to about 95° C. with externalhumidification at elevated pressures being required to maintain protonconductivity. As the membranes dry out at reduced humidity, protonconductivity deteriorates rapidly.

The industry is constantly looking for new membrane material thatconduct proton efficiently at reduced humidity levels. It would furtherbe desirable to provide membranes for PEM fuel cells with improved costand durability characteristics.

SUMMARY

New proton conducting membranes contain poly(arylene sulfide) polymers(PAS) bearing sulfonic acid groups. The membranes are prepared by thesolid state sulfonation of films comprising the poly(arylene sulfide)polymers, an olefinic polymer containing epoxy groups, and an elastomerthat improves the impact resistance, melt flowability, or flexibility ofthe film. A preferred poly(arylene sulfide) polymer is poly(p-phenylenesulfide). The membranes can be used in PEM fuel cells operating attemperatures above 95° C., or at low relative humidity. According tomethods of the invention, sulfonated poly(phenylene sulfide) (SPPS)films can be provided with a wide range of physical properties, whichdepend in part on the ion exchange capacity of the films. In particular,the degree or level of sulfonation can be tailored by adjusting reactionconditions such as temperature and time. The SPPS membrane materials canbe used as polyelectrolyte membranes in PEM fuel cells.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of three cells in a stack in anexemplary fuel cell system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

In one embodiment, the invention provides an polyelectrolyte membranecomprising an insoluble sulfonated film. The film is made of threepolymeric components, being A) a poly(arylene sulfide), B) an olefinicpolymer comprising epoxy groups, and C) an elastomer. The weight ratioof A/(B+C) is from 70/30 to 95/5, and the weight ratio B/C is from 10/90to 80/20 etc. The poly(arylene sulfides) are a known class ofthermoplastic polymer. In various embodiments, the poly(arylene sulfide)comprises polymeric reaction products of polyhalogenated aryl compoundswith a source of sulfur. In one embodiment, the polymer has a structureresulting from the reaction of para-dichlorobenzene and sodium sulfide,to form a preferred poly(p-phenylene sulfide).

The polyelectrolyte film can be produced as the reaction product of asulfonation reagent such as chlorosulfonic acid and an extruded filmcontaining components A) a poly(arylene sulfide), B) an olefinic polymercomprising epoxy groups, and C) an elastomer wherein the weight ratio ofA/(B+C) is from 70/30 to 95/5, and the weight ratio B/C is from 10/90 to80/20 etc. The polyelectrolyte film is insoluble in water, indicatingthe presence of some level of crosslinking in addition to theincorporated sulfonate groups. The sulfonate content of thepolyelectrolyte film, also referred to as the ion exchange capacity(IEC), is preferably at least 0.64 milliequivalents per gram (meq/g). Inpreferred embodiments, the sulfonate content in meq/g is 1.7 or greateror 1.9 or greater.

The invention also provides a membrane electrode assembly comprising acatalytic anode, a catalytic cathode, and a proton exchange membranedisposed between and in contact with the electrodes. The proton exchangemembrane is a polyelectrolyte film such as discussed above. Theinvention further provides fuel cells containing these membraneelectrode assemblies.

In another embodiment, the invention provides a method for making apolyelectrolyte film suitable for use in electrochemical applicationssuch as a fuel cell. The method comprises reacting a sulfonation reagentsuch as chlorosulfonic acid with an extended film comprising componentsA) a poly(arylene sulfide), B) an olefinic polymer comprising epoxygroups, and C) an elastomer. In one aspect, the method comprisesimmersing the extruded film in a solution of chlorosulfonic acid for atime and at a temperature sufficient to incorporate at least 0.64 meq/gSO₃H. The temperature and time of the reaction can vary over a widerange, depending upon the concentration of chlorosulfonic acid, thedegree of desired sulfonate incorporation, the desired turnaround, etc.For example, the temperature conveniently can range from about 20° C. toabout 50° C. and the time from about 1 to about 60 hours. In oneembodiment, the immersion of the film in the chlorosulfonic acidsolution takes place at room temperature for a time of 10 to 30 hours.

After immersing the film in the chlorosulfonic solution, the film isexposed to water to hydrolyze chlorosulfonic groups to sulfonic acidgroups. In one embodiment, the film is exposed to boiling water.

Fuel cells are provided according to the invention by using apolyelectrolyte film made by the above method as a proton exchangemembrane. In preferred embodiments, the sulfonate content of the protonexchange membrane is 1.7 meq/g or greater, the liquid water conductivityat 30° C. is greater than 0.1 S/cm, and the conductivity at 100° C. isgreater than 0.2 S/cm. In other preferred embodiments, the sulfonatecontent is 1.9 meq/g or higher.

A PEM fuel cell comprises an anode, a cathode, a proton exchangemembrane disposed between the anode and the cathode, an inlet forhydrogen fuel adjacent the anode, and an inlet for oxygen adjacent thecathode. The proton exchange membrane comprises a sulfonated film, asdescribed above, preferably having a sulfonate content of at least 0.64meq/g SO₃H. In various embodiments, the proton exchange membrane is aninsoluble sulfonated film made by the method described above.

Suitable films containing components A) a poly(arylene sulfide), B) anolefinic polymer comprising epoxy groups, and C) an elastomer aredescribed below and in U.S. Pat. No. 5,625,002 to Kadoi et al., thedisclosure of which is incorporated by reference.

Suitable poly(arylene sulfides) useful as component A) a poly(arylenesulfide) include polymers represented by formula (I)—[Ar—SO_(x)]_(n)—  (1)wherein Ar represents a divalent aromatic unit and x is an integer from0 to 2. When x is 1, the polymers may also be described as poly(arylene)sulfoxide. When x is 2, the polymers may be described as poly(arylene)sulfonyl polymers. Preferably x is 0, where the polymers arepoly(arylene sulfide) polymers, or PAS. Formula (1) represents polymersthat are made of alternating aromatic and sulfur containing units.Non-limiting examples of the divalent aromatic unit represented by Arincludes any of formulas (2)-(11)

Preferably Ar is a divalent phenylene unit represented by the formulas(2)-(4) and x is 0 in formula (1).

A preferred poly(arylene sulfide) polymer is poly(p-phenylene sulfide),also known as PPS. PPS has an idealized structure according to theformula

The poly(arylene sulfides) in general and PPS in particular can beprepared by reacting a polyhalogenated arylene compound with a source ofsulfur. In one embodiment, a PPS polymer can be formally represented asthe reaction product of a para-dihalobenzene such aspara-dichlorobenzene and a source of sulfide such as sodium sulfide. Tomake PPS and other members of the PAS class of polymers otherpolyhalogenated aromatics can be substituted for thepara-dihalobenzenes, and other sources of sulfur can be substituted forthe sodium sulfide. Non-limiting examples of halogenated aromaticsinclude para-dichlorobenzene; para-diodobenzene;1-chloro-4-bromobenzene; 1-chloro-4-iodobenzene; 2,5-dichlorotoluene;2,5-dichloro-p-xylene; 1-ethyl-4-isopropyl-2,5-dibromobenzene;1,2,4,5-tetramethyl-3,6-dichlorobenzene;1-butyl-4-cyclohexyl-2,5-difluorobenzene, and the like.

Mixtures of para- and meta-dichlorobenzene may be used with a source ofsulfide to prepare poly(arylene sulfide) useful to make thepolyelectrolyte membranes of the invention. In other embodiments, smallamounts of 1,2,4-trihaloarylenes, such as 1,2,4-trichlorobenzene may beused. Use of small amounts of the tri-substituted arylenes introducessome branching into the polymeric structure. It is also possible toproduce random copolymers containing paraphenylene sulfide and alkylsubstituted phenylene sulfides.

In various embodiments, the PAS can comprise up to 30 mole % ofrecurring units represented by any of the following structural formulae:

As described above, the kind of PAS used in the present invention is notparticularly critical, but in view of the affinity with an olefiniccopolymer described below, preferably PAS which has been subjected to adeionizing purification treatment to remove ionic species is used. Invarious embodiments, the ion content of PAS expressed as the sodiumcontent is not larger than 900 ppm, preferably not larger than 500 ppm.Non-limiting means for reducing the sodium content, include (a) an acidtreatment, (b) a hot water treatment, and (c) an organic solvent washingtreatment.

Acid treatment is carried out in the following manner. The PAS is dippedin or otherwise exposed to an acid or an aqueous solution of an acid,with appropriate stirring and heating. In a non-limiting example, apowdery PAS is immersed in an aqueous solution of acetic acid of pH 4,heated at 80° to 90° C., and stirred for 30 minutes. To remove theresidual acid or salt, the acid-treated PAS is afterwards washed withwater or warm water, preferably distilled water.

In a non-limiting example, hot water treatment is conducted by adding apredetermined amount of the PAS to a predetermined amount ofwater—preferably distilled water—and heating the thus-prepared mixtureunder stirring in a pressure vessel. Preferably the temperature of thehot water is at least 100° C., more preferably at least 120° C., mostpreferably higher than 150° C., and especially preferably higher than170° C. A large proportion of water to PAS is preferred, and in general,a bath ratio of not larger than 200 g of PAS per liter of water isselected.

To avoid degradation of the polymer, the treatment is preferably carriedout in an inert atmosphere. To remove the residual components,preferably the PAS that has been subjected to the hot water treatment iswashed several times with warm water, maintained at a temperature oflower than 100° C., more preferably at a temperature of at least 10° C.but lower than 100° C.

Alternatively or in addition, organic solvents not having an action ofdecomposing PAS can be used for washing PAS. Non-limiting examples ofsolvents include nitrogen-containing polar solvents such asN-methylpyrrolidone, dimethylformamide, dimethylacetamide,1,3-dimethylimidazolidinone, hexamethylphosporamide, and piperazinone;sulfoxide and sulfone solvents such as dimethyl sulfoxide,dimethylsulfone, and sulfolane; ketone solvents such as acetone, methylethyl ketone, diethyl ketone, and acetophenone, ether solvents such asdiethyl ether, dipropyl ether, dioxane, and tetrahydrofuran;halogen-containing hydrocarbon solvents such as chloroform, methylenechloride, ethylene dichloride, trichloroethylene, perchloroethylene,monochloroethane, dichloroethane, tetrachloroethane, perchloroethane,and chlorobenzene; alcohol and phenol solvents such as methanol,ethanol, propanol, butanol, pentanol, ethylene glycol, propylene glycol,phenol, cresol, polyethylene glycol, and polypropylene glycol; andaromatic hydrocarbon solvents such as benzene, toluene, and xylene. Ofthese organic solvents, N-methylpyrrolidone, acetone, dimethylformamideand chloroform are especially preferred. Further, these solvents can beused alone or as a mixture of two or more.

Washing with organic solvent is accomplished by immersing PAS in theorganic solvent and heating or stirring as appropriate. The washingtemperature for the organic solvent washing is not particularlycritical, and an optional temperature can be selected of from roomtemperature to about 300° C. The washing efficiency is increased with anelevation of the washing temperature, but in general, a satisfactoryeffect is obtained at a washing temperature of from room temperature to150° C.

Further, the washing can be carried out under pressure at a temperaturehigher than the boiling point of the organic solvent in a pressurevessel. The washing time is not particularly critical, and for abatchwise washing, a satisfactory effect is generally obtained if thewashing is conducted for at least 5 minutes. Alternatively, the washingcan be performed in a continuous manner.

In other embodiments, it is preferred to combine one or two of the othertreatments. For example when, a high-boiling-point organic solvent suchas N-methylpyrrolidone is used, the residual organic solvent can beeasily removed by washing with water or warm water after the organicsolvent washing, and preferably distilled water or deionized water isused for this washing.

In various embodiments, a satisfactory effect can be obtained bycarrying out the above-mentioned acid treatment or hot water treatmentalone. Alternatively a method can be adopted in which the acid treatmentis first carried out and the hot water treatment is carried outthereafter, or a method can be adopted in which the hot water treatmentis first carried out and then the acid treatment is carried out.

The epoxy group-containing olefinic polymer (B) used in the presentinvention is an olefinic polymer having an epoxy group in the side chainor main chain. Suitable polymers include olefinic polymers having aglycidyl group such as a glycidyl ester, a glycidyl ether or a glycidylamine in the side chain, and olefinic polymers having a double bondepoxy-oxidized. A preferred epoxy group-containing olefinic polymer is acopolymer of an α-olefin with a glycidyl ester of an α,β-unsaturatedacid. Examples of olefin include ethylene, propylene and butene-1. Theglycidyl ester of the α,β-unsaturated acid is represented by thefollowing formula:

wherein R stands for a hydrogen atom or a lower alkyl group having 1 to5 carbon atoms

Non-limiting examples, include glycidyl acrylate, glycidyl methacrylateand glycidyl ethacrylate. The content of the epoxy group in the epoxygroup-containing olefinic polymer (B) is preferably 0.1 to 30% byweight, especially 0.2 to 10% by weight. If the content of the epoxygroup is lower than 0.1% by weight, the desired effects cannot beobtained, and if the content of the epoxy group exceeds 30% by weight,gelation tends to occur at the melt kneading with PAS.

A minor amount of another olefinic monomer such as methyl acrylate,methyl methacrylate, acrylonitrile, styrene, vinyl acetate or vinylether can be copolymerized with the epoxy group-containing olefinicpolymer (B) used in the present invention, as long as the effects of thepresent invention still can be obtained.

As described above, the impact characteristics can be improved in a PAScomposition prepared by incorporating the epoxy group-containingolefinic polymer (B). Preferably, an elastomer (C) is incorporated intothe components A) and B) to improve the impact characteristics andenhance the melt flowability and flexibility.

The elastomer (C) used in the present invention preferably does notinclude an epoxy group and an acid anhydride group and has a flexuralmodulus not larger than 10,000 kgf/cm². The use of an elastomercontaining an acid anhydride group is not preferred because it wouldtend to raise the melt viscosity of the composition and reduce themoldability of the composition.

The elastomer (C) can be any elastomeric substance that provides thebenefits noted above. In preferred embodiments, the elastomer C) isselected from the group consisting of ethylene/propylene copolymers,ethylene/butene copolymers, ethylene/propylene/diene copolymers,hydrogenated styrene/butadiene/styrene block copolymers, copolymers ofethylene with acrylic acid, methacrylic acid or alkyl esters or metalsalts thereof, and polyamide elastomers. In various embodiments, otherelastomers, such as butadiene/styrene copolymer, butadiene/acrylonitrilecopolymer, polybutadiene, ethylene/vinyl acetate copolymer andplasticized polyvinyl chloride are less preferred.

The ethylene/propylene copolymer is a copolymer of ethylene andpropylene having a melt flow index of 0.1 to 50 g/10 min as determinedaccording to JIS K-7210, and preferably the ethylene content is 30 to95% by weight, more preferably 40 to 90% by weight.

A suitable ethylene/butene copolymer is a copolymer of ethylene andbutene-1 having a melt index of 0.5 to 50 g/10 min as determinedaccording to JIS K-7210, and preferably the ethylene content is 30 to95% by weight, more preferably 40 to 90% by weight.

A suitable ethylene/propylene/diene copolymer is a copolymer prepared byintroducing a diene compound into an ethylene/propylene copolymer, andthe iodine value as the factor indicating the quantity introduced of thediene compound is generally 5 to 30. As the diene compound to beintroduced, there can be mentioned hexadiene, norbornadiene, andethylidene norbornene.

A suitable hydrogenated styrene/butadiene/styrene block copolymer is acopolymer obtained by hydrogenating a part or all of the intermediatebutadiene blocks of a styrene/butadiene/styrene copolymer elastomer, forexample, according to the process disclosed in the specification of U.S.Pat. No. 3,413,323. The melt flow index of the copolymer is 0.5 to 100g/10 min, as determined according to JIS K-7210, and the styrene contentis preferably 5 to 60 mole %, more preferably 10 to 50 mole %.

Suitable copolymers of ethylene with acrylic acid, methacrylic acid, andalkyl esters in which the alkyl group has 1 to 5 carbon atoms, and metalsalts thereof, include ethylene/acrylic acid ester copolymers such as anethylene/methyl acrylate copolymer, an ethylene/ethyl acrylatecopolymer, an ethylene/propyl acrylate copolymer and an ethylene/butylacrylate copolymer, ethylene/methacrylic acid ester copolymers such asan ethylene/methyl methacrylate copolymer, an ethylene/ethylmethacrylate copolymer, an ethylene/propyl methacrylate copolymer, andan ethylene/butyl methacrylate copolymer, an ethylene/acrylic acidcopolymer and an ethylene/methacrylic acid copolymer, and their metalsalts such as Na, Zn, K, Ca and Mg salts.

Suitable polyamide elastomers include block copolymer elastomers havinghard segments of a polyamide component and soft segments of a polyethercomponent and/or a polyester component. Examples of the polyamidecomponent, include

NH—R^(I)—CO

_(n) and

NH—R^(II)—NHCO—R^(III)—CO

_(n) (in which R^(I,) R^(II) and R^(III) stand for an alkylene grouphaving 2 to 15 carbon atoms or a substitution product thereof). Examplesof the polyether component, include

OR

_(n) (in which R stands for an alkylene group having 2 to 15 carbonatoms or a-substitution product thereof). Examples of the polyestercomponent, include

OR^(I)—CO

_(n) and

O—R^(II)—OCO—R^(III)—CO

_(n) (in which R^(I), R^(II), and R^(III) stand for an alkylene grouphaving 2 to 15 carbon atoms or a substitution product thereof). Thepolyamide elastomer further includes random copolymers of nylon 6, nylon66, nylon 610, nylon 11 and nylon 12.

Of the foregoing elastomers (C), an ethylene/propylene copolymer, anethylene/butene copolymer, an ethylene/acrylic acid copolymer, anethylene/methacrylic acid copolymer, an ethylene/acrylic acid estercopolymer, and an ethylene/methacrylic acid ester copolymer arepreferred.

The mixing ratio among the PAS (A), the epoxy group-containing olefiniccopolymer (B), and the elastomer (C) is preferably within a range suchthat the (A)/[(B)+(C)] weight ratio is from 55/45 to 99/1, morepreferably from 70/30 to 95/5, and the (B)/(C) weight ratio is from 95/5to 5/95, more preferably from 80/20 to 10/90. If the ratio of thecomponent (A) is lower than about 55% by weight, the strength andrigidity of the composition tend to be lowered, and if the ratio of thecomponent (C) based on the sum of the components (B) and (C) is lowerthan 5% by weight, there tends to be little improvement of the meltflowability.

The method of preparing the film is not particularly critical. In oneembodiment, PAS (A), the epoxy group-containing olefinic polymer (B),and the elastomer (C), are melt-kneaded at a temperature higher than themelting point of the PAS in an extruder, and the resulting kneadedmixture is pelletized. Thereafter a film can be extended or solutioncast according to known techniques.

In general, preferably the melt-kneading temperature is higher than 280°C., to sufficiently melt the composition, and lower than 340° C. toprevent a thermal deterioration and gelation of the olefinic copolymer(B). Namely, preferably the melt-kneading temperature is 280° to 340° C.

A suitable film containing the three components A) a poly(arylenesulfide), B) an olefinic polymer comprising epoxy groups, and C) anelastomer is sold under the Ryton® 16C and XTEL-XE trade names byChevron-Phillips.

Because PAS and PPS have limited solubility in many solvents, it issometimes difficult or impossible to produce solution cast films ofcompositions containing them. However, the polymers are meltprocessable, so in a preferred embodiment extruded films are produced.Non-limiting examples of extruded films include 0.5 mil, 1 mil, and 2mil films. PAS is insoluble in concentrated sulfuric acid. but readilydisperses in oleum (fuming sulfuric acid consisting of 30% by weight SO₃in sulfuric acid) and in chlorosulfonic acid (ClSO₃H). The reaction witholeum leads to highly sulfonated polymers that are soluble in water,while reaction with ClSO₃H leads to the insoluble films of theinvention.

PAS reacts with chlorosulfonic acid to introduce the chlorosulfonylgroup (—SO₂Cl) onto aromatic rings of the polymers. The polymer-boundchlorosulfonyl groups are subsequently converted to sulfonic acid groupsupon hydrolysis, for example by exposing the sulfonated films to boilingwater for one hour. The products derived from reaction with thechlorosulfonic acid are insoluble. This is believed to be due in part toa crosslinking reaction involving the formation of sulfone groups(—SO₂—). The crosslinking provides advantages for the chlorosulfonationof preformed PAS films, because the crosslinking reaction reinforcesphysical properties and prevents the films from dissolving in water.

In various embodiments, a polyelectrolyte film suitable for use as aproton exchange membrane in a fuel cell is made by reacting an extrudedPPS film with chlorosulfonic acid. The films contain components A) apoly(arylene sulfide), B) an olefinic polymer comprising epoxy groups,and C) an elastomer, as described above. For example, preformed filmsare immersed in a solution of chlorosulfonic acid in a solvent such asdichloromethane or 1,2-dichloroethane. Immersion occurs for a time andat a temperature sufficient to incorporate a desired amount of sulfonateor chlorosulfonyl groups onto the aromatic rings of the polymer. In oneembodiment, at least 0.64 meq SO₃H/g is incorporated. After immersingthe film in the chlorosulfonic solution for a sufficient time, the filmis thereafter exposed to water. This step converts the chlorosulfonylgroups to sulfonic acid groups by hydrolysis. The hydrolysis step can becarried out at elevated temperatures, such as in boiling water, toprovide faster reaction kinetics.

After hydrolysis, the amount of sulfonic acid groups incorporated intothe film can be determined by conventional means, such as titration withsodium hydroxide to a pH of 7. The number of mL of sodium hydroxidesolution required to titrate the film to a pH of 7 is converted into meqSO₃H/g of film by conventional methods.

Useful polyelectrolyte membranes in general have liquid conductivitymeasured in S/cm at 30° C. or 100° C. that are comparable to those ofconventional perfluorosulfonic acids used as proton exchange membranesin fuel cells. For example, the commercially available proton exchangemembrane, Nafion 112 from DuPont exhibits a liquid conductivity at 30°C. of 0.129 S/cm at 30° C. and 0.259 S/cm at 100° C. Such conductivitymeasurements may be made as described by Zawodzinski et al., J. Phys.Chem. 95 (15) 6040 (1991). The membrane is conditioned in 100° C. waterfor 1 hour and the measurement cell is submersed in 25° C. deionizedwater during the experiment. The membrane impedance (real) is taken atzero imaginary impedance.

According to another embodiment of the invention, a fuel cell isprovided that contains the insoluble sulfonated film described above asa proton exchange membrane. Such fuel cells typically contain acatalytic anode, a catalytic cathode, and a proton exchange membranedisposed between the anode and cathode. The fuel cell also contains aninlet adjacent the anode for providing hydrogen fuel to the anode sideof the fuel cell, and an inlet adjacent the cathode for providingoxidant gas (oxygen) to the cathode.

Referring generally to FIG. 1, three individual proton exchange membrane(PEM) fuel cells according to one preferred embodiment of the presentinvention are connected to form a stack. Each PEM fuel cell hasmembrane-electrode-assemblies (MEA) 13,15,14, respectively, separatedfrom one another by electrically conductive, impermeable separatorplates 16,18, and further sandwiched between terminal separator plates20,22 at each end of the stack with each terminal plate 20,22 havingonly one electrically active side 24,26. An individual fuel cell, whichis not connected in series within a stack, has a separator plate, withonly a single electrically active side. In a multiple fuel cell stack,such as the one shown, a preferred bipolar separator plate 16 typicallyhas two electrically active sides 28,30 respectively facing a separateMEA 13,15 with opposite charges that are separated, hence the so-called“bipolar” plate. As described herein, the fuel cell stack has conductivebipolar separator plates in a stack with multiple fuel cells, howeverthe present invention is equally applicable to conductive separatorplates within a stack having only a single fuel cell.

The MEAs 13,15,14 and bipolar plates 16,18 are stacked together betweenaluminum clamping plates 32 at each end of the stack and the end contactterminal plate elements 20,22. The end contact terminal plate elements20,22, as well as working faces 28,30 and 31,33 of both bipolarseparator plates 16,18, contain a plurality of gas flow channels (notshown) for distributing fuel and oxidant gases (i.e., H₂ & O₂) to theMEAs 13,15,14. Nonconductive gaskets or seals (not shown) provide sealsand electrical insulation between the several components of the fuelcell stack. Gas-permeable conductive diffusion media 34 press up againstthe electrode faces of the MEAs 13,15,14. When the fuel cell stack isassembled, the conductive gas diffusion layers 34 assist in evendistribution of gas across the electrodes of the MEAs 13,15,14 and alsoassist in conducting electrical current throughout the stack.

An inlet for oxygen adjacent the cathode and an inlet for hydrogenadjacent the anode are also provided. Oxygen is supplied to the cathodeside 36 of each fuel cell in the stack from storage tank 40 viaappropriate supply plumbing 42 to provide an inlet for oxygen adjacentthe cathode, while hydrogen is supplied to the anode side 38 of the fuelcell from storage tank 44, via appropriate supply plumbing 46 to providean inlet for hydrogen adjacent the anode. Alternatively, air may besupplied to the cathode side 36 from the ambient, and hydrogen to theanode 38 from a methanol or gasoline reformer, or the like. Exhaustplumbing for the anode side 48 and the cathode side 50 of the MEAs13,15,14 are provided. On the cathode side, the plumbing defines an exitside. Gas flow into and out of the stack is typically facilitated byfans 60, such as those shown in the exemplary configuration of FIG. 1.Any means of transporting fluids into and out of the stack are feasible,and the configuration and number of fans shown is merely exemplary andnot limiting.

As shown in FIG. 1, the cathode effluent 50 is routed from the stack toa condenser 54, which serves to liquefy and recover the vapors in thecathode effluent stream 50. The liquids (e.g. water) are transported toa reservoir 56 for storage. The effluent stream 50 from the cathode hasa high concentration of vapor (water vapor, for example) due to thewater generated by the electrochemical reactions occurring within theMEA and any additional water introduced for cooling. The waterevaporates due to pressure and temperature conditions within the fuelcell. Preferably, the effluent stream is saturated with vapor (e.g. inthe case of water at approximately 100% relative humidity). As shown,the supply conduits 61 provide water to the cathode side of each MEA13,15,14 by interconnecting the reservoir 56 to the fuel cells in thestack. A pump (not shown) may optionally be included in the system tofacilitate the transport of the liquid from the reservoir 56 to thestack, or through other areas of the system.

The invention has been described above with respect to various preferredembodiments. Further non-limiting examples are given in the examplesthat follow.

EXAMPLES Example 1—Sulfonation of PPS Films with Chlorosulfonic Acid at25° C.

Dichloromethane (50 mL, 66 gm) and chlorosulfonic acid (between 0.7 and1.4 gms) are added sequentially to a wide mouth glass bottle (120 mLcapacity, 2 inch diameter). 10 mL of this solution are added todichloromethane (50 mL, 66 gms) in a wide mouth glass jar (410 mL, 3inch diameter). To this mixture is added a 1 mil (0.001 inch, 0.0025 cm)colorless film of Ryton® 16C or XTEL-XE (Chevron-Phillips) consisting ofa circle with a diameter of 2.75 inches and weighing between 0.14 and0.18 gm. The jar is sealed with a screw cap lid and the film is allowedto react for various amounts of time at 25° C. while being suspended inthe reaction solution. The insoluble colorless film is observed to turnblue-green and then black after several seconds of immersion in thereaction solution. After a variable time of reaction, the black film isthen added to distilled water (200 mL) and the film turned light yellow.The film is washed extensively with more water (about 2 liter) and thenboiled in water (250 mL) for about 1 hour. The film is then suspended in1 molar sodium chloride (220 mL) and the amount of sulfonation isdetermined by titration with 0.01 molar sodium hydroxide to a pH 7 endpoint. The amount of sulfonation (in meq/g SO₃H) with reaction-time is0.64 (1 hour), 1.27 (6.5 hours), 1.71 (16 hours), 1.86 (24 hours), 2.31(48 hours), and 2.6 (60 hours). The sulfonated film with 1.9 meq/g SO₃Hhad a liquid water proton conductivity of 0.131 and 0.337 S/cm at 33° C.and 100° C. respectively.

Example 2—Sulfonation Reaction at 40° C.

The reaction described above is repeated at 40° C. After 4 hours, theamount of sulfonation is determined by titration with 0.01 molar sodiumhydroxide to be 2.36 meq/g SO₃H. The physical properties of the film arebetter than those with membranes similar ion exchange capacities made at25° C.

Example 3

Polyelectrolyte films of Examples 1 and 2 are flexible enough to behandled and made into MEA's and fuel cells according to known methods.Comparable films made by chlorosulfonating a film containing componentA) but not components B) or C) are too brittle for subsequent use inmaking fuel cells.

Although the invention has been described above with respect to variouspreferred embodiments, the invention is not limited to the embodimentsdisclosed. Variations and modifications as will occur to those of skillin the art upon reading the disclosure are also included in the scope ofthe invention, which is limited only by the appended claims.

1. A polyelectrolyte membrane, comprising an insoluble sulfonated filmhaving a sulfonate content in meq SO₃H/g of at least 0.64, wherein thefilm comprises A) a poly(arylene sulfide), B) an olefinic polymercomprising epoxy groups, and C) an elastomer wherein the weight ratio ofA/(B+C) is from 70/30 to 95/5, and the weight ratio B/C is from 10/90 to80/20.
 2. A polyelectrolyte membrane, according to claim 1 wherein thepoly(arylene sulfide) comprises poly(p-phenylene sulfide).
 3. Apolyelectrolyte membrane according to claim 1, wherein the sulfonatecontent is 1.7 meq/g or greater.
 4. A polyelectrolyte membrane accordingto claim 1, wherein the sulfonate content is 1.9 meq/g or greater.
 5. Apolyelectrolyte membrane according to claim 1, comprising the reactionproduct of chlorosulfonic acid and an extruded film comprising thepoly(arylene sulfide), the olefinic polymer and the elastomer.
 6. A filmaccording to claim 5, wherein the poly(arylene sulfide) ispoly(p-phenylene sulfide).
 7. A membrane electrode assembly comprising aproton exchange membrane film according to claim
 1. 8. A fuel cellcomprising a membrane electrode assembly comprising a polyelectrolytemembrane according to claim
 7. 9. A film according to claim 1,comprising sulfone crosslinking groups.
 10. A method for making apolyelectrolyte film suitable for use in electrochemical applications,comprising reacting chlorosulfonic acid with a film comprising A) apoly(arylene sulfide), B) an olefinic polymer comprising epoxy groups,and C) an elastomer, wherein the weight ratio of A/(B+C) is from 70/30to 95/5, and the weight ratio B/C is from 10/90 to 80/20.
 11. A methodaccording to claim 10, wherein the poly(arylene sulfide) comprisespoly(p-phenylene sulfide).
 12. A method according to claim 10,comprising immersing the film in a solution of chlorosulfonic acid for atime and at a temperature sufficient to incorporate at least 0.64 meq/gSO₃H; and thereafter exposing the film to water.
 13. A method accordingto claim 12, comprising exposing the film to boiling water.
 14. A methodaccording to claim 12, wherein the temperature is from 20° C. to 50° C.,and for a time from 1 to 60 hours.
 15. A method according to claim 12,wherein the temperature is from 25° C. to 40° C.
 16. A method accordingto claim 12, wherein the temperature is room temperature.
 17. A methodaccording to claim 12, wherein the time is 10 to 30 hours.
 18. A fuelcell comprising a proton exchange membrane made by a process accordingto claim
 10. 19. A fuel cell according to claim 18, wherein the protonexchange membrane has a sulfonate content of 1.7 meq/g or greater.
 20. Afuel cell according to claim 18, wherein the proton exchange membranehas a conductivity at 30° C. greater than 0.1 S/cm.
 21. A fuel cellaccording to claim 18, wherein the proton exchange membrane has aconductivity at 100° C. greater than 0.2 S/cm.
 22. A fuel cellcomprising an anode; a cathode; a proton exchange membrane disposedbetween the anode and cathode. an inlet for hydrogen fuel adjacent theanode; and an inlet for oxygen adjacent the cathode; wherein the protonexchange membrane comprises a sulfonated film having an ion exchangecapacity of 0.64 meq/g or greater, wherein the film comprises A) apoly(arylene sulfide, B) an olefinic polymer comprising epoxy groups,and C) an elastomer, wherein the weight ratio of A/(B+C) is from 70/30to 95/5, and the weight ratio B/C is from 10/90 to 80/20.
 23. A fuelcell according to claim 22, wherein the poly(arylene sulfide) comprisespoly(p-phenylene sulfide).
 24. A fuel cell according to claim 22,wherein the ion exchange capacity capacity is 1.7 meq/g or greater. 25.A fuel cell according to claim 22, wherein the ion exchange capacity is1.9 meq/g or greater.
 26. A fuel cell according to claim 22, wherein theproton exchange membrane is made by a process comprising exposing anextruded film comprising components A) a poly(arylene sulfide, B) anolefinic polymer comprising epoxy groups, and C) an elastomer to asolution of chlorosulfonic acid and exposing the film to water.
 27. Afuel cell according to claim 22, wherein the proton exchange membranecomprises sulfonyl crosslinks.